Recently, energy storage technologies are receiving increasing attentions. As application areas expand to energies for mobile phones, camcorders and notebook PCs and further to electric vehicles, more systematic efforts are made for the researches and developments of electrochemical elements. Electrochemical element is gathering the most attention in this aspect, and especially, development of chargeable and dischargeable secondary batteries is becoming the focus of interest.
Diligent researches and studies gave rise to development of the electrochemical elements as the electrode active materials that have various significantly improved functions, such as the electrochemical element with improved output. Among the currently-available secondary batteries, lithium secondary battery developed in the early 1990s is coming into spotlight because of advantages such as higher operating voltages and superior energy density compared to conventional batteries such as Ni—MH, and the like.
Many companies are producing the electrochemical elements described above, but these have respectively varying safety aspects. For such electrochemical elements, evaluating and ensuring safety is of great importance. One of the most important issue to consider is that a user should not be injured in the event of malfunction of the electrochemical element, and for this purpose, safety rules are imposed for strictly regulating the ignition and fuming in the electrochemical element.
A separator of the electrochemical element plays an important role of passing electrolyte or ions while isolating a cathode and an anode to thus prevent shorts of the two electrodes, and it is thus required that the separator have various characteristics in view of electrical, chemical, and mechanical considerations.
For example, the separator has to be securely attached to the electrodes, and at the same time, has to have a reduced thickness for lightness and compactness of the electrochemical element without compromising a sufficient mechanical strength thereof.
Such separator can be formed by incorporating polyolefin-based porous polymer substrate, but this porous polymer substrate has a problem of insufficient electrolyte wettability. This problem is still experienced even when a porous coating layer containing a mixture of an inorganic particle and a binder polymer is formed on at least one surface of the porous polymer substrate, because while the porous coating layer may enhance electrolyte wettability, the electrolyte wettability of the porous polymer substrate is not enhanced.
Meanwhile, a method also has been suggested that the binder polymer is distributed with a gradient of concentration along a thickness direction of the separator, in which the binder polymer is phase-separated under a certain humidity condition so that more binder polymer is distributed on a surface of the separator where the electrode is adhered. However, while the above method may enhance adhesion between the separator and the electrode, it is difficult to expect an increase in electrolyte wettability of the porous polymer substrate.